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To my knowledge EEVEE is only supposed to use the GPU while rendering. But when I render something and inspect task manager my CPu usage is around 30-50%. As a comparison for Cycles with GPU render my CPU usage is around 15%.

Why does EEVEE use so much CPU processing power?

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    $\begingroup$ The "proper" way to answer this question is to run Blender under a profiler, do an EEVEE render, and then look at the profile to see where the time goes. Any other answer is going to be speculative at best. One obvious speculation: You have a compositor denoiser. That would run on the CPU not the GPU. EEVEE uses opengl and so anything opengl can't do on the GPU would also run on the CPU. $\endgroup$ Jun 12, 2022 at 15:31

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While the rendering itself is indeed only using the GPU because of it being an OpenGL rendering, it is wrong to assume that your CPU isn't involved by Blender in the entire process.

CPU power is still helpful to handle high complexity scenes as the geometry and modifiers are still prepared on the CPU before rendering each frame.

Source: Reference/Release Notes/2.80/EEVEE - Blender Developer Wiki

I don't know the details exactly, but I think that includes BVH, and also all the tasks on the system like writing the files and video encoding if you also output a video file (please don't do that for a final render).

Now, the reason why your specific case uses your CPU and GPU the way it does, can't be answered easily. It could be that your hardware is just bottle-necked by its CPU, or that your scene is not optimized. There have been many cases where people complained about their scene hitting seemingly insane render times only to be told that a simple two-clicks optimization fixes it all.

 I had a quick look at your scene and this is a typical case that screams “instances” to me. (...).
I just deleted all your geometry (...) added a simple plane with a particle system that instances the peg (...) thousands of times, many many more pegs than in your scene plus a building on every single one of it. (...)
File size went from 270MB to a mere 13MB, render preparation (BVH building etc.) takes under a second now and memory consumption during render is 90MB max.

Source: Building BVH is killing render time (Cycles, Eevee, and Redshift Maya Compared) - Lighting and Rendering - Blender Artists Community

Think of Eevee as a video game, it doesn't use "just" either of your GPU and CPU, it uses both for different things. Having bad performance only means that either one of these are hitting their limit or the software is hitting his limit. Most of the time you can either change the load to not hit any limits though.

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